Every gas, when cold, absorbs the same rays of light which it emits when incandescent. An immediate inference from this law is that the dark lines seen in the spectrum of the sun are caused by the passage of the light through gases either existing on the sun or forming the atmosphere of the earth. A second inference is that we can determine what these gases are by comparing the position of the dark lines with that of the bright lines produced by different gases when they are made incandescent. Hence arose the possibility of spectrum analysis, a method which has been applied with such success to the study of the heavenly bodies. So far as the general constitution of bodies is concerned, the canons of spectrum analysis are these: Firstly, when a spectrum is formed of distinct bright lines, the light which forms it is emitted by a transparent mass of glowing gas. Secondly, when a spectrum is entirely continuous the light emanates from an incandescent solid, from a body composed of solid particles, which may be ever so small, or from a mass of incandescent gas so large and dense as not to be transparent through and through. Thirdly, when the spectrum is continuous, except that it is crossed by fine dark lines, the body emitting the light is surrounded by a gas cooler than itself. The chemical constitution of this gas can be determined by the position of the lines. Fourthly, if, as is frequently the case, a spectrum is composed of an irregular row of bright and shaded portions, the body is a compound one, partly gaseous and partly solid. It will be seen from the preceding statement that, in reality, a mass of gas so large as not to be transparent cannot be distinguished from a solid. It is therefore not strictly correct to say, as is sometimes done, that an incandescent gas always gives a spectrum of bright lines. It will give such a spectrum only when it is transparent through and through.[C] [C] As this principle is not universally understood, it may be well to remark that it results immediately from Kirchoff’s law of the proportionality between the radiating and absorbing powers of all bodies for light of each separate wave-length. When a body, even if gaseous in form, is of such great size and density that light of no color can pass entirely through it, then the consequent absorption by the body of light of all colors shows that throughout the region where the absorption occurs there must be an emission of light of these same colors. Thus light from all parts of the spectrum will be emitted by the entire mass.